在《[原创]我想问人大同学，是教科书错了，还是我错了?[原创]我想问人大同学，是教科书错了，还是我错了?》一文中，有些概念需要澄清。

A stock is a quantity measured at a given point in time, whereas a flow is a quantity measured per unit of time.
存量衡量一个给定时点上的数量。
流量衡量每一个单位时间内的数量。

Income is what you earn from working plus what you receive in interest and dividends. It is a flow-that is, it is expressed per unit of time: weekly income, monthly income, or yearly imcome. Getty was once asked what his income was. Getty answered, "$1000." What he meant but did not say was, "$1000 per minute."
收入是指工作报酬加上获得的利息和红利。它是流量——也就是说，它可以表示成单位时间变量：周收入，月收入或年收入。格蒂曾被人问及他的收入是多少，格蒂的回答是“1000美元”，他的意思是每分钟的收入是1000美元，虽然他没有明确表示。

Saving is that part of after-tax income that is not spent. It is also a flow. If you save 10% of your income and your income is $3000 per month, then you save %300 per month. Savings(plural) is sometimes used as a synonym for wealth-the value of what you have accumulated over time.
储蓄指税后收入中没有被花费的那一部分，它也是一个流量。如果你的月收入是3000美元，并且把收入的10%进行储蓄，那么你每月将储蓄300美元。储蓄（英文复数）有时用作财富的同义词——一段时期以来所积累的价值。

Your financial wealth, or simply wealth, is the value of all your financial assets minus all your financial liabilities. In contrast to imcome or saving, which are flow variables, financial wealth is a stock variable. It is the value of wealth at a given moment in time. At a given moment in time, your cannot change the total amount of your financial wealth.You can change it only over time, as you save or dissave, or as the value of your assets change.
金融财富，或者简单一点，只是财富指所有金融资产减去金融负债的价值。与收入和储蓄这两个流量不同，金融财富为存量，它所表示的是给定时刻的财富价值。在给定时间点，不能改变金融资产的总量。你只能随着时间的推移才能做到这一点，比如进行储蓄或者动用储蓄，以及资产的价值改变。

Economists use investment to refer to the purchase of new capital goods.
经济学家使用的投资指购买新资本物品。

The amount of capital in the economy is a stock; the amount of investment is a flow.
经济中的资本量是存量；投资量是流量。

The goal of GDP is to summarize in a single number the dollar value of economic activity in a given period of time.
There are two ways to view this statistic. One way to view GDP is as the total income of everyone in the economy.Another way to view GDP is as the total expenditure on the economy’s output of goods and services.
GDP的目的是用一个单一的数字来概括某一给定时期经济活动的美元价值。
有两种方法可以估计这一统计数据。一种方法是把GDP作为经济中的所有人的总收入。另一种方法是把GDP作为对经济中产品与服务的产出的总支出。

GDP is probably the most important flow variable in economics: it tells us how many dollars are flowing around the economy’s circular flow per unit of time. When you hear someone say that the U.S. GDP is $10 trillion, you should understand that this means that it is $10 trillion per year. (Equivalently, we could say that U.S. GDP is $317,000 per second.)
GDP也许是经济学中最重要的流量变量：它告诉我们每一单位时间内经济的循环流程中有多少美元在流动。当你听到某个人说，美国的GDP是10万亿美元时，你应到知道，这意味着每年是10万亿美元。（同样，我们可以说，美国的GDP是每秒钟317000美元。）

因此，宏观经济学中的收入、储蓄、投资，指得全部是流量。尤其是，翻译过来的储蓄(saving)（宏观中用中文写出来的所有“储蓄”）指的是流量，而不是savings.
而Y=C+I=C+S，全部是流量。

参考书目：
1. N.Gregory Mankiw, Macroeconomics.
2. Olivier Blanchard, Macroeconomics.

另外，在国家统计局的网站上，有国民经济核算的指标解释，可供参考：

http://www.stats.gov.cn/tjzd/tjzbjs/t20020327_14293.htm

其中写道：总储蓄 指可支配总收入扣除最终消费后的余额。